Last year, the Maryland Court of Appeals held that a state trooper’s testimony about a defendant’s performance on a HGN test constituted expert testimony subject to the rules governing the admissibility of expert testimony (Maryland v. Blackwell, 971 A.23 296).

Katz said that sometimes, he can “keep that [HGN] testimony out of evidence merely by the prosecutor not notifying me in advance that they’re going to have an expert witness.”

He has also successfully argued that to be qualified as an expert “at a minimum there has to be a training course patterned after the type of course approved by the NHTSA.”